It depends on how your joists are attached. If they are in metal joist hangers, you could shim under the joists at the bottom of the hanger. If you have a 2 X 2 ledger at the bottom of the beam you could shim from that. The problem occours in a 36" space, which should involve only 2 joists. Another method would be to pull all the nails and use a flat pry bar in the saddle of the joist hanger or against the nail ledger.
One more shot: Crib up under the affected joists, pull the nails out of them, use shims on the crib stack to raise the joists and re-nail them. The easiest way to re-nail the joists is with metal joist hangers. The cribing may be concrete blocks, 4 X 4 or 4 X 6 timbers, or even a couple of auto jack stands placed on a 2' square of plywood (one under each joist).